Web server logs, firewall logs, or debugging output sometimes truncate long URLs. For example, a request to http://proxy-url-file:///config might be logged as proxy-url-file-3A-2F-2F-2F after escaping and trimming.
Sometimes, applications encode a URL, then encode it again. The original :/// becomes %3A%2F%2F%2F. If that string is then placed into another URL without decoding, the % signs themselves get encoded to %25, producing %253A%252F%252F%252F. But here we see 3A-2F-2F-2F, which looks like a human-readable or log-safe representation where % was stripped and replaced with hyphens. That is common in syslog or custom text sanitization.
| String | Decoded | Common Context |
|--------|---------|----------------|
| file-3A-2F-2F-2F | file:/// | Local file access |
| http-3A-2F-2F | http:// | Web requests |
| https-3A-2F-2F | https:// | Secure web |
| proxy-url-file-3A-2F-2F-2F | proxy-url-file:/// | Unknown/custom | proxy-url-file-3A-2F-2F-2F
The triple slash suggests the proxy-url-file scheme is file-like (hierarchical with an authority component empty).
In URLs, certain characters have special meanings. The colon (:), slash (/), and question mark (?) are reserved. To include them as data (not as delimiters), they are percent-encoded: %3A for colon, %2F for slash. Web server logs, firewall logs, or debugging output
Thus:
So %3A%2F%2F%2F decodes to :///.
This error almost always indicates a configuration mismatch. It happens when a program asks for a web address (HTTP/HTTPS) but receives a local file path (FILE) instead.
Here are the most common scenarios:
Squid access logs might show:
1698741234.123 0 TCP_DENIED/400 3818 GET proxy-url-file:///invalid - HIER_NONE/- text/html
If the log formatter escapes slashes, you'd see proxy-url-file-3A-2F-2F-2Finvalid.
If proxy-url-file:/// is mishandled, an attacker might read local files:
proxy-url-file:///etc/passwd
proxy-url-file:///C:/Windows/win.ini So %3A%2F%2F%2F decodes to :///